1. Field of the Invention
This invention relates to force transducers using cantilever beams having stress sensitive portions carrying electrical strain sensing means, and relates also to the application of such transducers to differential pressure and process pressure transmitters for flow, level, and pressure measurement and transmission.
2. Description of the Prior Art
Differential pressure and process pressure transducers and transmitters for flow, level, and pressure measurement and transmission are known in the prior art. Examples of such transmitters are shown in the following U.S. Pat. Nos., which are incorporated herein by reference: 3,712,143 -- Weaver et al, 3,559,488 -- Weaver, 3,894,435 -- Schimada et al, 3,479,739 -- Stedman, 3,461,416 -- Kaufman, 3,780,588 -- Whitehead et al.
The force transducer shown in U.S. Pat. No. 3,712,143 is a so-called cup type unit with piezoresistive strain sensing elements formed in situ on, that is, diffused into the surface of, a silicon diaphragm. The latter forms a flexible wall of the cup. The piezoresistive elements are connected into an electrical bridge network to provide an electrical signal that varies in accordance with the magnitude of the differential or process pressure to which the silicon diaphragm is subjected. Since the diaphragm must be constructed of extremely pure silicon or other suitable semiconductor material, the transducer is restricted as to the thinness and flexibility of its diaphragm. Also, where the diaphragm must be relatively large, as is required in applications involving measurements in the lower pressure ranges, for example, the manufacturing cost of the structure is very high.
U.S. Pat. No. 3,559,488 shows in FIG. 1 a transducer wherein strain gauge elements are attached to a pressure-responsive diaphragm. This patent shows also, in FIG. 2, a transducer wherein strain gauge elements are attached to a beam which is actuated by a pressure-responsive diaphragm. U.S. Pat. Nos. 3,894,435; 3,479,739; and 3,461,416 also show transducers wherein strain gauge elements are attached to beams which are actuated by pressure-responsive diaphragms. Although these constructions free the pressure sensors, that is, the diaphragms, from the material and cost restrictions noted above and resulting from the diffusion of the strain elements into the diaphragms, the resulting transducers are subject to undesired hysteresis which results from the attachment of the strain gauge elements to the diaphragms or beams. Moreover, these transducers are subject to additional undesired hysteresis which, I have discovered, results from the combining in the transducers of materials having different internal viscosities.
The force transducer of U.S. Pat. No. 3,780,588 embodies a non-cup structure wherein piezoresistive elements are diffused into the surface of a silicon beam having one end mounted in a fixed manner. The other end of the beam is moved in accordance with the expansion and contraction of a bellows which is exposed to the differential pressure or process pressure under measurement. While this arrangement frees the pressure sensor, that is, the bellows, from the restrictions noted above, and frees the transducer from the undesired hysteresis which results from the attachment of strain gauge elements, this arrangement is still subject to the noted additional undesired hysteresis which results from the combining of materials having different internal viscosities, e.g., the materials of the beam and the bellows.
Further problems encountered in the use of the beams shown in the five last-mentioned patents are those of providing a precise fulcrum for the flexed section of the beam, providing a mounting that is unaffected by temperature changes, and providing a way to attach a spring or other force transmitting means to the movable end of the beam without creating concentrations of stress, and thereby non- uniform strain patterns, in the flexed section of the beam.